An airbag device that inflates to deploy an airbag between occupants seated on the rear seats of a vehicle for protecting the occupants from colliding with each other is proposed in Japanese Unexamined Utility Model Registration Application Publication No. Hei 3-121156. The airbag device disclosed in Japanese Unexamined Utility Model Registration Application Publication No. Hei 3-121156 is provided with a side airbag for inflating to deploy between the door and the occupant in a side collision and with an airbag for restraining the rear seat occupant by deploying in the middle between the rear seats in a side collision.
An airbag device disclosed in Japanese Unexamined Utility Model Registration Application Publication No. Hei 3-121156 is housed in a movable armrest storable in a rear seat back. Therefore, the airbag device basically operates when the armrest is kept horizontal. However, unless the airbag device is designed to operate when the armrest is in a stored state, a function of the airbag device as an occupant restraint system is significantly depreciated. Accordingly, the airbag system disclosed in Japanese Unexamined Utility Model Registration Application Publication No. Hei 3-121156 includes two airbags and a set of airbags which are switched to operate depending on whether or not the armrest is stored in the seat back.
However, the airbag device causes such problems as complicated gas line as well as necessity for two airbags.